Open cell porous biocompatible foams have been recognized to have significant potential for use in the repair and regeneration of tissue. Early efforts in tissue repair focused on the use of amorphous biocompatible foam as porous plugs to fill voids in bone. Brekke, et al. (U.S. Pat. No. 4,186,448) described the use of porous mesh plugs composed of polyhydroxy acid polymers such as polylactide for healing bone voids. Several attempts have been made in the recent past to make TE scaffolds using different methods, for example U.S. Pat. Nos. 5,522,895 (Mikos) and 5,514,378 (Mikos, et al.) using leachables; U.S. Pat. Nos. 5,755,792 (Brekke) and 5,133,755 (Brekke) using vacuum foaming techniques; U.S. Pat. Nos. 5,716,413 (Walter, et al.) and 5,607,474 (Athanasiou, et al.) using precipitated polymer gel masses; U.S. Pat. Nos. 5,686,091 (Leong, et al.) and 5,677,355 (Shalaby, et al.) using polymer melts with fugitive compounds that sublimate at temperatures greater than room temperature; and U.S. Pat. Nos. 5,770,193 (Vacanti, et al.) 5,769,899 (Schwartz, et al.) and 5,711,960 (Shikinami) using textile-based fibrous scaffolds. Hinsch et al. (EPA 274,898) described a porous open cell foam of polyhydroxy acids with pore sizes from about 10 to about 200 .mu.m for the in-growth of blood vessels and cells. The foam described by Hincsh could also be reinforced with fibers, yarns, braids, knitted fabrics, scrims and the like. Hincsh's work also described the use of a variety of polyhydroxy acid polymers and copolymers such as poly-L-lactide, poly-DL-lactide, polyglycolide, and polydioxanone. The Hincsh foams had the advantage of having regular pore sizes and shapes that could be controlled by the processing conditions, solvents selected, and the additives.
However, the above techniques have limitations in producing a scaffold with a gradient structure. Most of the scaffolds are isotropic in form and function and lack the anisotropic features of natural tissues. Further, it is the limitation of prior art to make 3D scaffolds that have the ability to control the spatial distribution of various pore shapes. The process that is described to fabricate the microstructure controlled foams is a low temperature process that offers many advantages over other conventional techniques. For example the process allows the incorporation of thermally sensitive compounds like proteins, drugs and other additives with the thermally and hydrolytically unstable absorbable polymers.
Athanasiou et al. (U.S. Pat. No. 5,607,474) have more recently proposed using a two layer foam device for repairing osteochondral defects at a location where two dissimilar types of tissue are present. The Athanasiou device is composed of a first and second layer, prepared in part separately, and joined together at a subsequent step. Each of the scaffold layers is designed to have stiffness and compressibility corresponding to the respective cartilage and bone tissue. Since cartilage and bone often form adjacent layers in the body this approach is an attempt to more clearly mimic the structure of the human body. However, the interface between the cartilage and bone in the human body is not a discrete junction of two dissimilar materials with an abrupt change in anatomical features and/or the mechanical properties. The cartilage cells have distinctly different cell morphology and orientation depending on the location of the cartilage cell in relation to the underlying bone structure. The difference in cartilage cell morphology and orientation provides a continuous transition from the outer surface of the cartilage to the underlying bone cartilage interface. Thus the two layer system of Athanasiou, although an incremental improvement, does not mimic the tissue interfaces present in the human body.
Another approach to make three-dimensional laminated foams is proposed by Mikos et al. (U.S. Pat. No. 5,514,378). In this technique which is quite cumbersome, a porous membrane is first prepared by drying a polymer solution containing leachable salt crystals. A three-dimensional structure is then obtained by laminating several membranes together, which are cut to a contour drawing of the desired shape.
One of the major weaknesses of the prior art regarding three-dimensional porous scaffolds used for the regeneration of biological tissue like cartilage is that their microstructure is random. These scaffolds, unlike natural tissue, do not vary in morphology or structure. Further, current scaffolds do not provide adequate nutrient and fluid transport for many applications. Finally, the laminated structures are not completely integrated and subjected to delamination under in vivo conditions.
Therefore, it is an object of the present invention to provide a biocompatible, bioabsorbable foam that provides a continuous transitional gradient of morphological, structural and/or materials. Further, it is preferred that foams used in tissue engineering have a structure that provides organization at the microstructure level that provides a template that facilitates cellular invasion, proliferation and differentiation that will ultimately result in regeneration of functional tissue.